I have been playing around with a Celestron First Scope (FL = 300 mm; Apert = 76 mm), my iPhone 6 & a Svbony Metal Universal Camera Mount Telescope Digital Camera Adaptor (which is very fiddly to set up!).

I took out my 2x Barlow lens, inserted a Celestron 20 mm FL len with adapter/iPhone attached only to discover a black blob (please see dreadful images attached) in the centre of the image. I could also see the blob if I looked directly into the EP & looked 'around' the FoV, but it wasn't centrally placed. I cleaned everything I could get my hands on, but the problem remained: no amount of tiddling with the focus got rid of it. After faffing around & trying a Meade 40 mm FL EP & getting the same result, I decided to try again plus Barlow lens - the blob 'disappeared'. I've never seen this blob before, but then never used the scope without the Barlow.

Am I right - probably not - that this is some kind of weird, optical, focussing issue/abberation (sorry, not a very technical explanation) with 'long' FL lenses plus such a short 'scope? I've done an internet search & the nearest thing I can find is that the blob is a 'image' of the secondary mirror.

Did O-level physics 40-ish years ago, so have forgotten much of the optics stuff - I'm so embarrassed

I'm keen to learn anything astro-related so can anyone shed some light on this mysterious blob?

Many thanks in advance.

BTW: I do a have a larger Newtonian reflector in storage awaiting the dark winter skies The little scope is for playing with &looking at the moon

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_________________"Where the telescope ends, the microscope begins. Which of the two has the grander view?" Victor Hugo

Here's a daft reply. There was a scifi film in the 1950s called "The Blob", but I can't remember how they got rid of the wobbly alien. Maybe the desperate townsfolk added custard and ate it. Sorry to seem to be frivolous Sue, but it's a bit of a mystery. Maybe it's due to the exit pupil being too big and producing the equivalent of the "kidney bean" effect.

Is this a reflector you are using? So no diagonal? From what you say I think this is the case. Looks to me like its a focusing problem, the background is not in focus and the blob should not be seen at true focus.

Low magnification on very small instruments with a secondary mirror obstruction , either in Newtonian or Maksutov-type configuration, always produce this very annoying shadow. Using a higher magnification improves the problem but the image will them be poor. Even on a large reflector with a secondary obstruction it is impossible to use a very low power eyepiece without this annoying blob appearing. Regards maf.

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